1. Field of the Invention
This invention relates to a horizontal output circuit in use with a television receiver, and more particularly, to the improvements of a horizontal output circuit for a television receiver, which has a correcting property of the called horizontal pincushion distortion, and is free from the raster distortion.
2. Description of the Related Art
When considering the deflection of cathode ray tubes (CRT) used for, for example, normal color television receivers and display monitors, the radius of curvature of the fluorescent screen of the CRT is not equal to the distance from the deflection center to the screen. The noncoincidence of these centers causes the raster on the screen to be distorted in a pincushion fashion. This type of raster distortion is usually corrected by increasing the horizontal amplitude toward the center of the vertical scanning. To be more specific, this correction is made by parabolically amplitude modulating an envelope of the horizontal deflecting current at the vertical periods.
The circuit arrangement of a horizontal output circuit with the pincushion correcting property will be described. A drive pulse signal varying at the horizontal periods is applied to the base of a horizontal output transistor. A damper diode and a resonant capacitor are coupled across the collector-emitter path of the transistor. A horizontal deflecting coil and a DC blocking capacitor called the S-distortion correcting capacitor are connected in parallel with a series circuit of the primary coil of a first transformer. The collector of the transistor is connected to a DC power source, through the primary coil of a second transformer. The first transformer is for correcting the horizontal pincushion distortion, with the drive coil to which a parabolic drive current at the vertical periods is fed. To form the drive current, the voltage in the vertical deflecting coil is integrated by an integrating circuit for wave shaping, and finally fed to the drive coil. The second transformer as a flyback transformer is connected in such a way that the secondary coil supplies a high voltage through a rectifier circuit to the anode of the CRT.
In the horizontal output circuit thus arranged, when the parabolic current at the vertical periods is fed to the primary coil of the drive coil, inductance of the primary coil connected in series to the horizontal deflecting coil varies in a parabolic fashion, thereby amplitude modulate the envelope of the horizontal deflection current at the vertical periods.
The horizontal output circuit using the correcting transformer, however, involves some problems to be solved. For a large correction, the inductance of the primary coil of the first transformer must be varied greatly. This results in increase of a ripple component contained in the high voltage, which comes through the high voltage rectifier in the secondary coil of the flyback transformer. This ripple causes the raster distortion. Further, the horizontal deflection current may saturate the correcting transformer. The saturation leads possibly to degradation of the horizontal linearity performance and nonuniformity of the correcting amount in the horizontal direction.
When a white signal as a video signal is received, the load current in the secondary coil of the flyback transformer (second transformer) increases, so that the increased amount of the current flows through the primary coil. This adversely effects the horizontal deflection current. The adverse effect appears in the reproduced picture such that when a cross pattern, for example, is received, the vertical bar following reproduction of the white horizontal bar is horizontally bent. A voltage drop resulting from the load current increase the called trapezoidal distortion. In the distortion, the bottom line extend outward, giving the shape of a trapezoid.
Another horizontal output circuit with a correcting property of horizontal pincushion distortion is disclosed in U.S. Pat. No. 3,906,305 (Japanese Patent Publication (Kokoku) No. 39102/57). The arrangement of this circuit is shown in FIG. 1. In the figure, a pulse signal at the horizontal periods is applied to the base of horizontal output transistor Tr. The collector-emitter path of this transistor is coupled in parallel with a series circuit consisting of first and second damper diodes D.sub.1 and D.sub.2, which are connected with the polarity as shown, and another series circuit consisting of first and second resonant capacitors Cr and Cr'. Further, capacitor Cr is connected across a series circuit consisting of horizontal deflecting coil Ly and S-distortion correcting capacitor Cs. Capacitor Cr' is connected across a series circuit consisting of coil L.sub.1 and capacitor C.sub.1. Power source from DC power source E is supplied to the collector of transistor Tr via the primary coil Lo of transformer T.sub.1. Modulating source M is connected across capacitor C.sub.1. Modulating source M supplies a vertical pulse signal as received, to the base of transistor Tr', via drive stage Dr'. A control signal appearing across the collector-emitter path of transistor Tr' varies in a parabolic fashion. The control signal is applied across capacitor C.sub.1, to control horizontal deflection current iy for correcting the horizontal pincushion distortion.
The operation of the horizontal output circuit thus arranged will be described referring to FIGS. 2A through 2C. At the start of the scanning period t.sub.1 to t.sub.4, damper diodes D.sub.1 and D.sub.2 are conductive, so that the circuit operation enters a damping phase or period ranging from t.sub.1 to t.sub.2. During the damping period, the voltages across first and second S-distortion correcting capacitors Cs and C.sub.1 are respectively applied to coils Ly and L.sub.1. These voltages cause saw-tooth wave currents to flow these coils, respectively. The current flowing through coil Ly is horizontal deflecting current i.sub.y. Before the operation reaches the mid-point of the scanning period, a control signal (on-pulse) is supplied to the base of transistor Tr, and then transistor Tr is turned on. At the mid-point, the direction of the coil current is reversed. When current i.sub.y is larger than the current i.sub.l flowing through coil L.sub.1, the current i.sub.y flows through transistor Tr. A difference of these currents i.sub.y -i.sub.1 flows through diode D.sub.2. At this time, diode D.sub.1 is not conductive. In the reverse case where current i.sub.1 is larger than current i.sub.y, a current amounting to current i.sub.l flows through transistor Tr. A difference between these current i.sub.l -i.sub. y flows through diode D.sub.1. Diode D.sub.2 is not conductive. At the end of the scanning period, transistor Tr is turned off, and diodes D.sub.1 and D.sub.2 thus far conductive are not conductive. Operation enters a blanking phase or period ranging from t.sub.0 to t.sub.1. Therefore, the current flowing into transistor Tr flows into resonant capacitors Cr and Cr'. A voltage appears across the series circuit consisting of resonant capacitors Cr and Cr'. The waveform of this voltage resembles a sinusoidal waveform. As this voltage returns to zero, diodes D.sub.1 and D.sub.2 are both conductive, and the next scanning starts.
As already mentioned, the time period t.sub.0 to t.sub.1 is the blanking period. Time period t.sub.1 to t.sub.2 is the damping period. Time period t.sub.1 to t.sub.4 is the scanning period. Assuming that an AC current flowing through the primary coil of flyback transformer T.sub.1 is i.sub.p, a DC current flowing therethrough is i.sub.in, an AC current flowing through coil L.sub.1 is i.sub.1, the sum of these currents i.sub.p +i.sub.l +i.sub.in is a saw-tooth wave current, as shown in FIG. 2A. When a rectangular wave video signal of white is supplied to the CRT display, a white peak current flows through the secondary coil of flyback transformer T.sub.1, so that DC current flowing through the primary coil of transformer T.sub.1 increases. At time point t.sub.2 before time point t.sub.3 that the on-pulse as shown in FIG. 2B is applied to the base of horizontal output transistor Tr, if the current (i.sub.p +i.sub.l +i.sub.in) flowing into damper diode D.sub.2 becomes zero, diode D.sub.2 is turned off during the period t.sub.2 to t.sub.3 (discontinuous period). Discontinuous pulse e.sub.1 as shown in FIG. 2C is observed in the voltage Vcr' across second resonance capacitor Cr'. The pulse e.sub.1 pulls down the average value Vcs of the voltage Vcs across S-distortion correcting capacitor Cs. This is because Vcs+Vcr'183 =E. Accordingly, when the white rectangular wave signal is supplied to the CRT display, and the signal level exceeds a predetermined value, the horizontal output circuit responds to this and voltage Vcs gradually drops. The gradual voltage drop gives an inversed trapezoidal distortion, or a keystone distortion, in which the bottom line compressed inward. The shape of the distorted figure is the inversion of that as caused by the horizontal output circuit using the correcting transformer.
The horizontal output circuit contains first and second circuitries. The first circuitry consists of horizontal deflecting coil Ly, first resonant capacitor Cr, first damper diode D.sub.1, which are coupled in parallel. The second circuitry consists of coil L.sub.1, second resonant capacitor Cr', and second damper diode D.sub.2, which are also coupled in parallel. The scanning periods of the first and second circuitries must be terminated simultaneously. Therefore, the constants of these circuitries must be selected so as to satisfy Ly.times.Cr=L.sub.1 .times.Cr'. Practically, however, it is very difficult to select those constants so as to satisfy the above relation and to secure the screen linearity correction (pincushion distortion correction).
As described above, in the conventional horizontal output circuit using the distortion correcting transformer, when a large correction is required, the raster distortion due to the high-voltage ripple occurs. Additionally, the correcting transformer may be saturated, so that the screen linearity is damaged. Further, when a white signal is received, the bending of the image or the trapezoidal distortion occur.
In the circuit that the first and second circuitries are connected to the horizontal output transistor, the modulating source M at the vertical periods is used in place of the correcting transformer, when it receives a white signal, a discontinuous phenomena occurs, so that the inversed trapezoidal distortion occurs or the circuit constant selection is difficult.